The optimal arginine requirement in diets for juvenile humpback grouper, Cromileptes altivelis

Abstract

An 8-week feeding trial was conducted to evaluate the effects of dietary arginine (Arg) levels on growth performance, gut micromorphology, oxidation resistance and immune responses of humpback grouper, Cromileptes altivelis. Seven isoenergetic (340 kcal per 100 g of dry matter), isoproteic (50% of dry matter) and isolipidic (10% of dry matter) experimental diets were formulated to contain 1.74%, 2.42%, 3.14%, 3.58%, 4.06%, 4.54% and 5.21% dietary Arg levels. Triplicate groups of 12 fish (average initial body weight: 6.55 ± 0.1 g) were fed to apparent satiation by hand twice daily (08:00 h and 16:00 h) in an indoor recirculating seawater system which was consisted of 21 glass tanks (length 60 cm × width 45 cm × height 50 cm) and mechanical and biological water filters. After the feeding trial, all remaining fish were challenged by 2.5 mg Cu(II)·L−1, and survival rates were recorded for 12 h. Results showed that weight gain% (WG%), protein efficiency ratio (PER) as well as protein productive value (PPV) of fish were improved with the increments in dietary Arg from 1.74% to 3.58%, and thereafter, values of these parameters displayed a declining trend as dietary Arg level continued to rise from 3.58% to 5.21%. Fish fed 3.14% and 3.58% dietary Arg had lower FCR and daily feed intake (DFI) than fish fed other dietary Arg levels. Quadratic regression analysis of WG% against dietary Arg levels indicated that optimal dietary Arg level for maximum growth of humpback grouper was 3.39% of dry matter (6.78% of dietary protein). Hepatosomatic index (HSI) was reduced as dietary Arg level increased. Fish fed 3.58% dietary Arg had higher whole-body protein content than fish fed other dietary Arg levels. Expression of hepatic insulin-like growth factor-I (IGF-1), target of rapamycin (TOR), S6 kinase1 (S6K1) and hypothalamus growth hormone receptor (GHR) in fish fed 3.14% and 3.58% dietary Arg was higher than that in fish fed other dietary Arg levels. Gut micromorphology was significantly influenced by dietary Arg levels. After the exposure to 2.5 mg Cu(II)·L−1 water for 12 h, fish fed 3.14% and 3.58% dietary Arg had higher survival ratios and expression of NF-E2-related factor 2 (Nrf2) and heat shock proteins genes 70 (HSP70) in head kiney. In serum, fish fed 3.58% dietary Arg had higher superoxide dismutase (SOD) and catalase (CAT) activities and immunoglobulinM (IgM) concentrations than fish fed other dietary Arg levels. In conclusion, the optimal dietary Arg requirement for maximal growth of humpback grouper was estimated to be 3.39% of dry matter (6.78% of dietary protein), and suitable dietary Arg supplementations improved growth performance, gut micromorphology, oxidation resistance as well as immunity of this species.